The present invention relates to a precursor fiber bundle to be processed into a carbon fiber bundle, a process for producing the precursor fiber bundle, a carbon fiber bundle, and a process for producing the carbon fiber bundle. In more detail, the present invention relates to a precursor fiber bundle to be processed into a carbon fiber bundle, which is low in production cost, excellent in productivity, and which experiences less fiber breakage and fuzz generation, and which can be transformed into a sub-tow having an optimum formation for supplying to a process for producing a carbon fiber bundle. This invention also relates to a process for producing the precursor fiber bundle, to a carbon fiber bundle prepared from the sub-tow, and to a process for producing the carbon fiber bundle.
Furthermore, the present invention relates to a precursor fiber bundle comprising an acrylic polymer processed into a carbon fiber bundle, a process for producing the same, a carbon fiber bundle obtained from the precursor fiber bundle, and a process for producing the carbon fiber bundle.
Conventional precursor fiber bundle to be processed into a carbon fiber bundle is made of an acrylic polymer. The fiber bundle filaments may number from 3,000 to 20,000, and have a fineness of from 1,000 denier to 24,000 denier with small occurrences of fiber breakage and fuzz. It has been used for production of carbon fiber bundles having high strength and high modulus.
The precursor fiber bundle comprising an acrylic polymer processed into a carbon fiber bundle have been widely used as reinforcing fibers for components in the field of aerospace, sports, etc. The conventional carbon fiber bundle has been mainly examined to enhance its strength and the elastic modulus of carbon fibers. Specific items of examination include degree of crystalline orientation and densifying property of the precursor fibers, single filament breakage, fuzz, adhesion between filaments, acceleration of stabilization of the precursor fibers, etc.
The utilization of carbon fibers is being expanded at a rapid pace into general industrial fields including automobiles, civil engineering, architecture, energy, compounds, etc., and it is advantageous to supply a raw fiber bundle (precursor fiber bundle) to be processed into a carbon fiber bundle as a multifilament having improved strength and elastic modulus, at lower cost, and with increased productivity.
However, the raw fiber bundle (precursor fiber bundle) intended to be processed into a carbon fiber bundle is actually produced as a multifilament and wound on a drum or bobbin, and supplied in this style to a process for producing a carbon fiber bundle. Due to restrictions in the process of producing the carbon fiber bundle, particularly restriction of thickness (fineness) of the precursor fiber bundle in the stabilizing process, the rate of productivity has been kept remarkably low.
That is, the precursor fiber bundle comprising an acrylic polymer, processed into a carbon fiber bundle, is heated in an oxidizing atmosphere having a temperature of from 200.degree. C. to 350.degree. C. for stabilizing prior to carbonizing treatment. The stabilization treatment causes oxidization and cyclization, but since it generates heat, the heat stored in the fiber bundle becomes an important factor. If the heat stored in the fiber bundle is excessive, fiber breakage and adhesion between filaments occur. So, the stored heat must be kept low enough to prevent this.
Accordingly, a precursor fiber bundle having excessive thickness cannot be supplied into the stabilizing furnace. In industrial production the precursor fiber bundle is accordingly restricted in thickness (fineness). The restriction unfortunately keeps productivity low and is an obstacle in reducing production cost.
Producing a thermoplastic synthetic fiber bundle as a raw fiber bundle to be processed into a spun yarn or a non-woven fabric, not as a precursor fiber bundle to be processed into a carbon fiber bundle, is disclosed in Japanese Patent Laid-Open (Kokai) No. 56-4724. In this process, a tow running into a crimping apparatus is divided by dividing pins located close to the entrance of the crimping apparatus. A plurality of divided sub-tows are simultaneously supplied into the crimping apparatus, so that the plurality of sub-tows may be crimped as a whole, to be collected as one crimped tow capable of being potentially divided into crimped sub-tows later. However, if this process is applied to production of a precursor fiber bundle intended to be processed into a carbon fiber bundle, fiber breakage occurs often. This lowers the grade of the product since it is necessary to divide into a plurality of sub-tows a precursor fiber bundle having a fineness of not less than 300,000 denier in which filaments are engaged with each other by mutual oblique crossing and are closed up each other. This also adversely affects the production of carbon fibers.